Development relating to silicon photonics wherein optical functional elements are formed on a silicon substrate has been progressing due to reasons related to the superiority of processing technology, to industrial ripple effects, and to miniaturization of elements. Silicon itself does not provide a light emitting mechanism and, therefore, it is necessary to input light from the outside.
One of the techniques for this is a hybrid integration system wherein a spot size converter is provided on the silicon waveguide side, a semiconductor laser with a spot size converter is prepared as an external light source and the end surfaces of the two are placed in opposition to each other by adjusting the respective spot sizes so as to be the same size for optical coupling.
In this case it has been proposed that the silicon waveguide be made to be gradually narrower so as to increase the effusion of light and, thus, to increase the beam spot size (see Patent Document 1). A spot size converter that is tolerant to variations in manufacture has also been proposed (see Patent Document 2).
Furthermore, it has been proposed to make the end surface of the waveguide into a semi-cylindrical lens form in order to increase the efficiency in coupling with the semiconductor laser (see Patent Document 3).
FIGS. 11A to 11C are a diagram and graphs illustrating the structure in the vicinity of the end surface of a conventional optical waveguide coupler through which light enters and exits. FIG. 11A is a schematic perspective diagram, FIG. 11B illustrates a distribution of the Si composition ratio in the stacking direction, and FIG. 11C illustrates a distribution of the refractive index in the stacking direction. As illustrated in FIG. 11B, a core layer 62 is made of SiOx (x<2) where the composition ratio x is constant. Accordingly, as illustrated in FIG. 11C, the refractive index distributes in a step function manner for a lower clad layer 61 and an upper clad layer 63 made of SiO2. When a laser beam from a semiconductor laser 64 is incident on this core layer 62, the laser beam transmits through the core layer 62.
FIGS. 12A and 12B are graphs illustrating the coupling efficiency of a conventional optical waveguide coupler. FIG. 12A is a graph illustrating the results of a simulation of a distribution of light intensity, where the right half is a simplified copy of the left half. FIG. 12B is a graph illustrating the intensity of light in the z direction, that is to say, in the direction in which the light propagates. It can be seen from FIG. 12A that the beam spreads. As illustrated in FIG. 12B, significant attenuation is observed in the intensity of the light in the direction in which the light propagates. Here, the arrow in FIG. 12B indicates the position from which light enters. This is the result of two-dimensional BPM calculation relative to the coupling with a semiconductor laser having a beam spot size in an elliptical form that is wide in the horizontal direction.